1. Field of the Invention
This invention relates to an intrinsic safety barrier having utility in electronic process control systems comprising a central station and a plurality of remote stations connected to said central station and to each other by a single, time-shared, high density, coaxial cable or data highway.
2. Description of the Prior Art
In electronic process control systems comprising a central station and a plurality of remote stations, the central station generally is located in a control house and the remote stations are located in the field at strategically located areas. Control systems of this type find extensive application in industrial processes involving petroleum refineries and chemical plants. While the several stations are generally not located in hazardous or unsafe areas, it is frequently necessary for the interconnecting cable or data highway to pass through such hazardous areas. Incendivity considerations concerning the data highway thus become of extreme importance. This is because of an inherent danger of accidental or other destruction of the highway and release of an explosion-igniting arc. A flame-proof or explosion proof installation of the data highway is an answer, although an unsatisfactory one, to the problem.
Hazardous and non-hazardous areas have been defined in the literature, and have been referred to as "Zones." Zones 0, 1 and 2 are defined by the International Electrochemical Commission, as follows:
Zone 0 -- An area or enclosed space within which any flammable or explosive substance, whether gas, vapor or volatile liquid, is continuously present. PA1 Zone 1 -- An area within which any flammable or explosive substance, whether gas, vapor or volatile liquid is processed, handled or stored, and where during normal operations an explosive or ignitable mixture is likely to occur. PA1 Zone 2 -- An area within which any flammable or explosive substance, whether gas, vapor or volatile liquid, although processed or stored, is so well under conditions of control that the production of an explosive or ignitable concentration is only likely under abnormal conditions.
These terms are generally accepted in the industrial market areas, with the exception of the United States where the Zones are called Divisions. Division I includes Zones 0 and 1 and Division II includes Zone 2. Hereinafter the term Division will be used.
The concept of intrinsic safety in electronic process control systems is known in the prior art. The concept is based upon the restriction of electrical energy levels in field mounted components that may be located in a Division I area, for example, under normal or abnormal conditions, to a point well below that which could produce an explosion-igniting arc. Intrinsic safety in such systems thus is dependent upon the electrical circuit design rather than upon arc-containing explosion-proof enclosures, or other external physical protective measures.
A form of intrinsically safe barrier that has been proposed in the prior art and which depends upon electrical circuit design is the so-called Zener diode barrier. This barrier comprises a Zener diode which is connected in shunt to the interconnecting cable or highway and an associated series resistor and a fuse. Zener diode barriers have been subject to certain disadvantages, however, resulting from the capacitance of the Zener diode and the power and voltage loss introduced by the series resistance which must be relatively large. As a result of the relatively high capacitance of the Zener diode, higher frequency signals are shunted whereby the frequency of the intelligence or data signals that can be transmitted on the data highway is limited. This seriously limits the information density, that is, the amount of information that can be transmitted in a given period of time. A further and particularly important disadvantage of the Zener diode and resistor arrangement as an isolating barrier is that the connection between the safe and unsafe areas is galvanic. This is because with such a barrier there is a direct electrically conductive connection between the safe and unsafe areas. As a result, with such a barrier, the data highway would have to be grounded at the central station and at each of the remote stations where such a barrier is employed. Differences in ground potential along the data highway tend to establish circulating currents in the cable sheath which cause the introduction of spurious signals into the highway cable that interfere with the data signals.
The intrinsic safety barrier of the present invention provides a practicable alternative to a flame-proof installation of the data highway, which would be prohibitively expensive, and avoids the foregoing disadvantages of the Zener diode barrier whereby an improved intrinsically safe data communications system is achieved.